The present invention relates to a resin-bonded rare earth-iron-boron magnet and to a method of manufacturing thereof.
Rare earth magnets, particularly those containing rare earth and cobalt, such as RCo.sub.5 and R.sub.2 Co.sub.17, wherein R stands for at least one of yttrium and a rare earth element, have been known to the art. These permanent magnets, however, have energy products ((BH)max) approximately on the order of 30MGOe at most. And they require ample use of relatively expensive Co.
Therefore, relatively inexpensive rare earth-iron-boron magnets have been proposed in recent years to take the place of rare earth-cobalt magnets. Rare earth-iron-boron magnets are described in U.S. Pat. No. 4,597,938, U.S. Pat. No. 4,601,875, and U.S. Pat. No. 4,664,724, for example. They are composed of constituent elements of Nd, Fe and B. Such magnets are highly advantageous because they enjoy a reduction in cost due to the use of Fe and are producible with (BH)max exceeding 30MGOe.
Also, resin-bonded magnets, in which magnetic powder is bonded by resin, have an advantage in that they can be fabricated in a rich variety of shapes. Therefore, a resin-bonded rare earth-iron-boron magnet has been desired. A sintered magnet shows magnetic properties as a result of the overall sintered mass. However, a resin-bonded magnet requires that each particle of the powder has excellent magnetic properties, since the powder particles of a resin-bonded magnet are only bonded with a resin. Therefore, sintered magnet techniques can not simply be applied to a resin-bonded magnet.
Until this invention, the production of a resin-bonded magnet required the use of a powder obtained by melt-spinning, which is reported in European Patent Publications 108474, 125752 and 5 144112, for example. The magnet obtained by melt-spinning is naturally isotropic. However, a magnet desirably has anisotropic magnetic properties, because such a magnet can have a larger (BH)max than a magnet with isotropic properties. When a powder obtained by the melt-spinning method is used, an anisotropic resin-bonded magnet can be produced by the method comprising steps of:
(i) producing a powder by melt-spinning, wherein the powder has isotropic magnetic properties; PA0 (ii) hot-pressing the resultant powder in a desired shape; PA0 (iii) subjecting the hot-pressed body to hot plastic deformation thereby forming an anisotropic bulk; PA0 (iv) pulverizing the bulk into an anisotropic powder; and PA0 (v) bonding the anisotropic powder with a resin.
The melt-spinning method itself is complicated. Furthermore, for producing an anisotropic magnet, complicated steps such as (ii) and (iii) above are additionally needed. Therefore, an easy method for forming resin bonded magnets, to replace the melt-spinning method, has been sought. For example, a method using a casted alloy or a sintered alloy is reported in Japanese Patent Application Disclosures (KOKAI) 59-219904 and 62-102504 for example. However, use of a powder obtained by pulverizing a cast alloy or a sintered alloy has not yet been practical for resin-bonded rare earth-iron-boron magnets. This is because the magnetic powder used for the production of a resin-bonded magnet is required to have a particle size on the order of submillimeters. However, when pulverized to the level of submillimeters, the casted alloy or a sintered alloy suffers from a sharp drop of coercive force (iHc) as reported in Materials Letters: vol. 4 No. 5,6,7 (1986) 304. The coercive force may be improved to a certain extent by using a sintered alloy having an increased rare earth element content and subjecting the powder of the sintered alloy to an aging treatment. This procedure, however, has a disadvantage that the individual particles of the powder coalesce and the clusters resulting from the coalescence must be pulverized again, as reported in IEEE Trans. Magn. MAG-23 (1987) 2512. The pulverization so performed the second time degrades the coercive force again and induces deterioration of the rectangular property of the B-H hysteresis loop.